1. Field of the Invention
The present invention relates to a method and a system for controlling a numerical control cutting machine for cutting a work.
2. Description of Related Art
It is common for cutting operation such as numerically controlled cutting operation to feed a rotating cutting tool on a cutting surface of a metal work along a specified path. During the cutting operation, the rotating cutting tool possibly encounters a change in cutting load. In order, for example, to cut an object work having a number of free curved surfaces such as a metal mold for press-forming a body component of a vehicle, it is popular to use a ball end mill as a three-dimensional rotating cutting tool. Even this ball end mill is apt to be so often applied with load in excess.
In order for the rotating cutting tool to be free from being applied with cutting load in excess, it is conventionally practiced to control a relative feed speed of the rotating cutting tool according to a change in cutting load that is detected during cutting. It has been proposed in, for example, Japanese Unexamined Publication No. 61-30355 to prevent cutting edges of a rotating cutting tool from being damaged due to an excessive cutting load. This is realized by starting adaptive control in response to detection of contact of the rotating cutting tool with an object work and reducing a tool feed speed at a start of the adaptive control.
It is advantageous to protection of a rotating cutting tool from being applied with cutting load in excess to vary the tool feed speed according to cutting load during cutting operation. However, although the cutting load is detected by monitoring a load current on a main spindle motor, the load current is affected by various factors, so that it is hard to regard the load current as representing true cutting load and, in consequence, it is hard to securely protect the rotating cutting tool from being applied with cutting load in excess. Further, regarding the NC machining systems, cutting is performed under the control of program telling a cutting procedure, a tool path, cutting conditions and the like. This makes it hard to change control variables according to a change in load current during cutting. Therefore, in order to prevent the rotating cutting tool from being damaged, it is essential to set a constant tool feed speed so as to fit part of an object work where the rotating cutting tool is applied with heaviest cutting load. In this case, the rotating cutting tool is fed at a comparatively low tool feed speed even over part of the object work where the rotating cutting tool is applied with light cutting load.
It is therefore an object of the present invention to provide a method for controlling a numerical control cutting machine which can prolong service life of a rotating cutting tool by preventing the rotating cutting tool from being applied with loaded in excess.
It is another object of the present invention to provide a method for controlling a numerical control cutting machine that can save cutting time.
The aforesaid objects of the present invention is achieved by a cutting machine control method of controlling a numerical control cutting machine equipped with a rotating cutting tool, such as a ball end mill, for cutting a surface of a work.
According to an aspect of the present invention, the cutting machine control determines a required stock-removal volume to be removed from an object work along a tool path on the basis of a blank configuration of the object work and variably controls a tool feed speed of the rotating cutting tool along the tool path according to the required stock-removal volume so that the rotating cutting tool removes a predetermined stock-removal volume of material from the object work per unit time.
Because the stock-removal volume of rotating cutting tool per unit time is substantially proportional to cutting load applied to a rotating cutting tool, it is not too much to say that observing a stock-removal volume of the rotating cutting tool per unit time is equivalent to observing a cutting load indirectly. As the cutting machine control of the invention controls a tool feed speed so that a stock-removal volume becomes a specified or target stock-removal volume, when the target stock-removal volume is appropriately established, the rotating cutting tool is prevented from being applied with a cutting load in excess. This contributes to a long service life of the rotating cutting tool. That is, to bring a stock-removal volume per unit time to the predetermined stock-removal volume implies a change in tool feed speed according to a required stock-removal volume from a work along a tool path. Therefore, each cutting edge of the rotating cutting tool is prevented from being applied with a cutting load in excess by decreasing a tool feed speed for part of the work that has a large required stock-removal volume. In addition, the rotating cutting tool is fed at an increased tool feed speed for part of the work that has a small necessary stock-removal volume, this contributes to making a cutting time for the entire pan of a work as short as possible.
According to another aspect of the present invention, the cutting machine control determines a required stock-removal volume to be removed from an object work along a tool path on the basis of a blank configuration of the object work and variably controls a tool feed speed of the rotating cutting tool along the tool path according to the required stock-removal volume so that the rotating cutting tool removes a predetermined stock-removal volume of material from the object work per one revolution.
Because the stock-removal volume of cutting edge per one revolution is substantially proportional to cutting load applied to a cutting edge, it is not too much to say that observing a stock-removal volume of one cutting edge per one revolution is equivalent to observing a cutting load applied to the cutting edge indirectly. When controlling fir tool feed speed of the rotating cutting tool so that the cutting edge removes the specified stock-removal volume per one revolution, there occurs no significant difference in cutting load applied to one cutting edge irrespective of differences in the number of cutting edge and/or speed of rotation of the of the rotating cutting tool. Therefore, each catting edge of the rotating cutting tool is prevented from being applied with a cutting load in excess irrespective of the number of cutting edges and speed of rotation of the rotating cutting tool. This contributes to a long service life of the rotating cutting tool In addition, the rotating cutting tool is fed at an increased tool feed speed for part of the work that has a small necessary stock-removal volume, it is prevented that a cutting time for the entire work is made long.
The required stock-removal volume may be determined for each of a plurality of component path sections into which the whole length of the tool path is divided. In this event, the rotating cutting tool is fed at a constant tool feed speed along the component path section so that the rotating cutting tool removes the required stock removal volume from the object work for the component path section when the rotating cutting tool cuts the object work along each the component path section with the predetermined stock-removal rate per unit time.
The rotating cutting tool is fed at a constant tool feed speed over each component path section but not over the entire tool path. The rotating cutting tool is slowed down for a component path section along where the required stock removal volume is large and speeded up for a component path section where the required stock removal volume is small. This contributes to making a cutting time for the entire part of a work as short as possible.
In the case where an end mill or end mills are used, when the end mill is fed to approach a cutting surface of the object work at a tool feed angle with respect to the cutting surface in the component path section, the component path section is divided into a plurality of subsections and the rotating cutting tool is fed at a tool feed speed that is decreasingly varied in steps for each the subsection, the subsection being shorter in length for the tool feed angle of 90xc2x0 than for the tool feed angle other than 90xc2x0.
The circumferential speed a chamfer of the rotating end mill falls smaller as a distance from the axis decreases and comes to zero at the center and, in consequence, the cutting performance is lower at the center than the periphery 1 edge. This implies that when vertically feeding the end mill to a cutting surface of a work at a relatively high feed speed, the cutting edges are applied with excessively high cutting load when coming into contact with the cutting surface. This leads to damages and/or breakage of the cutting edges. Whereas, the cutting machine control of the present invention gradually decelerates the rotating cutting tool in steps while the rotating cutting tool is vertically fed with respect to a cutting surface of a work, so as to prevent the cutting edges being applied with cutting load in excess when the cutting edges are brought into contact with the cutting surface and thereafter.
The tool feed speed for an instant component path section may be replaced with the tool feed speed for the preceding component path section when a rate of tool feed speed change for the instant component path section relative to the tool feed speed for the preceding component path section is smaller than a specified rate.
The required stock-removal volume for a component path section that is determined on the basis of information on cutting surface configuration of a work possibly includes an error. In addition, when the tool feed speed varies for each component path section, while the control of tool feed speed becomes somewhat troublesome, movement of the cutting machine (the rotating cutting tool and/or a work) is not always smooth and cutting loading applied to the rotating cutting tool at a point where the tool feed speed changes suddenly. This not only increasing a cutting time but also applies the rotating cutting tool with a cutting load in excess. As a result, the cutting surface roughed and cutting accuracy is lowered. Whereas, the cutting machine control of the present invention continues cutting without changing a tool feed speed when a rate of tool feed speed change is low. As a result of this, the cutting machine control is simple and prevents the cutting machine from being jerky in operation that realizes cutting stability.
The specified rate of tool feed speed change may be changed larger with an increase in the tool feed speed for the previous component path section, Specifically, part of a work for which the rotating cutting tool is fed at a lower tool feed speed applies the rotating cutting tool with a light cutting toad and provides an ignorable change in cutting load only even when the rotating cutting tool is actually fed at a speed different from the determined tool feed speed. In this point of view, the cutting machine control controls a change in tool feed speed as small as possible by increasing the specified rate of tool feed speed change while the tool feed speed is high with an aim to provide cutting stability. From the other way, part of a work for which the rotating cutting tool is fed at a higher tool feed speed applies the rotating cutting tool with a heavy cutting load and provides a large change in cutting load when the rotating cutting tool is actually fed at a speed different from the determined tool feed speed. In light of this, the cutting machine control controls precisely the tool feed speed while the tool feed speed is low.
The cutting machine control system of the present invention comprises means for determining a required stock-removal volume to be removed from an object work along a tool path on the basis of data on the tool path and data on a blank configuration of the object work and means for determining a tool feed speed of the rotating cutting tool along the tool path according to the required stock-removal volume so that the rotating cutting tool removes a predetermined stock-removal of material from the object work volume per unit time, and means for feeding the rotating cutting tool at the tool feed speed.
The cutting machine control system prevents the rotating cutting tool from being applied with a cutting load in excess while making it possible to make a cutting time for the entire part of a work as short as possible. This contributes to a long service life of the rotating cutting tool.
A control medium for use with the cutting machine control method and system is recorded with a computer program to execute the functions of routing a tool path along which the rotating cutting tool is fed to cut an object work, dividing the tool path into a plurality of component path sections determining a required stock-removal volume to be removed from an object work for each the component path section on the basis of the information on the tool path and information of a blank configuration of the object work and a tool feed speed of the rotating cutting tool along the component path section according to the required stock-removal volume so each cutting edge of the rotating cutting tool removes a predetermined stock-removal volume of material from the subject work per one revolution of the rotating cutting tool, and storing data of the tool feed speed of the rotating cutting tool for each the component path section.
Data on the tool fed speed can be provided using the recording medium by entering information on rotating cutting tools, works, a tool path and a blank configuration of a cutting surface of the work into the computer.